250K potentiometer for tone control with preamp ?

[Jim Bonnell]

I'm putting a cheap Artec piezo under the saddle along with a lipstick pickup in the neck position on a guitar I'm just finishing up. The 50K tone pot on the preamp isn't working right. It's very scratchy and cuts out. Can I replace it with a 250K pot I already have or do I need to find one of the same value?

[Mark Swanson]

50K or lower is good, 250K is too high. How do you have this wired Jim?

[Greg Robinson]

Also, what value capacitor are you using?

[Jim Bonnell]

The Artec preamp had the pots mounted to the board. There isn't enough clearance it the guitar body to mount it the way I wanted so I removed the pots and ran extension wires to them. I have the two pickups run to separate jacks so they can be fed to different amps. I've never done this before so I have to admit I'm not real sure of what I'm doing. I wanted to be able to blend the two with only one amp or perhaps a Y cable from both jacks to one amp, but am trying to keep it simple at this point. As far as cap size I don't know what it is. I assume it's built into the board. I used alligator clips as heat sinks when I removed the pots and soldiered on the wires, but it's possible I over heated something that's causing my problem now. I didn't think the 250K would work from what I read on All Parts website, but I was hoping to use what I already had. Guess I'll order a 50K in the morning. Thanks to both of you for the help.

[Greg Robinson]

Jim, it might not be that the pot is damaged, it might be the solder joints. Are the joints nice and shiny and clean and properly flowed?

[Jim Bonnell]

I'll check again but I believe so. They seemed pretty good at the time. When I turn the tone knob the sound gets very scratchy at certain points.

[Mark Swanson]

Is the pot properly grounded? The shell of the pot as well as the ground at one of the pins.

[Jim Bonnell]

Yes. I checked and the ground is good and all the soldier joints look good. I'll try posting picture of the guitar later tonight. It's all done except for the pot problem.

[Mark Swanson]

The back of the pot is also grounded? I have heard problems just like this when there's a ground problem, and I looked in there 1000 times without seeing it.

[Jim Bonnell]

I though you might be right Mark so I took the cover off again and checked. I did ground the case and I put the tester to it just to making sure it was grounded well and it checked out fine.

[Greg Robinson]

Well, sounds like the pot may have been damaged Jim, but you might try spraying it out with tuner cleaner before you give up on it, although best to replace it if it's started to get scratchy this early in its life.

[Jim Bonnell]

That was the first thing I tried. I may have over heated it taking it loose from the board then soldiering new wires on. If you set the tone in the right spot it works fine. Just don't try and turn it. Thanks to both of you for all the help. I love this place!

[Jim Bonnell]

Well I learned something today I'm just not sure what it is. I had a spare preamp so I decided to switch it out. After I removed the old pots from the body just on a whim I decided to plug everything in and see if there was a difference with the old pots out of the body but still wired up. Sure enough it sounded great. I checked all connections then reinstalled the pots. Same old scratchy crappy sound. Took them back out and it was fine. I had copper foil tape on the inside of the guitar top where the pots fasten in and I cut it away from that area and reinstalled the pots and all is well. Why would the tape grounding it be a problem? It isn't on the pots for the magnetic pickup. Anyhow problem solved I just don't understand why. Most likely a very simple reason.

[Mark Swanson]

i had my suspicions that the pot was never bad. Was the copper strip grounded also? Or was its ground connection made by the pots themselves as they bolted down? Your problem definitely has something to do with the ground, and in an active circuit things may be a bit different with the grounds between different parts of the circuit, such as the volume and tone controls. I am just making guesses, the fact is you solved things and I am glad!

[Jim Bonnell]

No the copper had no ground other than the one through the pots. What a difference in the sound! It's 100% better. Thanks for all the help. I guess you don't always having to understand everything you learn.

[Greg Robinson]

Glad you got it sorted Jim!
With a single supply preamp (9V battery), parts of the circuit will be at a half bias point (~4.5V). It's possible one of the controls is on this "floating" ground, so when the foil shielding you used shorted the pots ground levels (ground is just a reference point, it doesn't have to be "0" volts) together, you got current flow between them. Any DC voltage across a pot in an audio path will make scratchy sounds as you turn it. Anyway, just some speculation.

[Jim Bonnell]

Thanks for the explanation Greg. Not only did the scratching disappear, the overall sound improved tremendously. Thanks again to you both for all the help.

[David King]

Floating grounds are much more common in modern preamp designs but they require a lot of care during installation to avoid shorting out half the preamp and causing all the symptoms you experienced. It's just a matter of time before we all get caught up with these developments.